1. Field of the Invention
The present invention relates to a temperature-responsive hydraulic control system for an automatic transmission and, more particularly, to a control system for hydraulic servos to apply/release frictional engagement elements in the transmission mechanism of an automatic transmission.
2. Related Art
In an automatic transmission, the feed of oil pressure to the hydraulic servos for applying/releasing the frictional engagement elements is controlled by the hydraulic control system. Specifically, the oil pressure, i.e., the line pressure, as fed through a manual valve, is raised by the pressure regulator valve and fed to the hydraulic servo in accordance with predetermined application characteristics, i.e. raised from the initial state of zero oil pressure to the application pressure, ultimately to the line pressure. For the control of this application pressure, an accumulator is usually used together with the pressure regulator valve. However, the accumulator must have a large capacity and, accordingly, occupies a large space in the hydraulic control system. For this reason, our Japanese Application Laid-Open No. 142203/1994, discloses controlling the application pressure exclusively by the pressure regulator valve. In this system, a signal pressure from an electronically controlled solenoid valve is applied to the pressure regulator valve for controlling the feed of the oil pressure to the hydraulic servos which actuate the frictional engagement elements. In this manner the application pressure is directly controlled by the pressure regulator valve.
In our previous system, described above, if the signal pressure is applied directly to the spool (valve member) of the pressure regulator valve abutting the plunger, the spool exhibits "vibration," in which it repeatedly overshoots and falls short of the desired pressure regulating position, so that the pressure, as regulated by the pressure regulating valve, pulsates. In order to suppress this pulsation, an orifice, having an opening sized to suppress the pulsation of the oil pressure, is usually arranged in the oil passage which feeds the signal pressure to the pressure regulating valve. A similar construction is disclosed in the aforementioned Japanese published application.
The orifice (throttle) arrangement described above is effective for the case in which the flow resistance (viscosity) of the oil is substantially constant. As a matter of fact, however, the viscosity of the oil changes with temperature so that the velocity of the oil flowing through the orifice drops with lower oil temperatures as the viscosity increases. If the oil passage is throttled by an orifice of predetermined size, as in the aforementioned construction, the response of the spool to a change in the signal pressure is excessively delayed at lower oil temperatures, thus causing a response delay in the oil pressure to be regulated. In a serious case, shift shock may result from the delay in the timing of application of the frictional engagement element. The prior art has failed to give special consideration to the change in the pressure regulating characteristics of the pressure regulator valve deriving from a change in temperature.